Asphalt is a viscoelastic semi-solid bituminous material derived from the distillation residue of crude petroleum. Typically, asphalt has an atmospheric equivalent boiling point of at least 350.degree. C., and more generally above 500.degree. C. Because it has good adhesive and weathering characteristics, it is widely used as a binder or cement for stone or rock aggregate in pavement construction. The asphalt cement typically comprises only about 5% of the mixture of asphalt and aggregate. If, because of the nature of the crude oil distilled, the asphalt is too soft at ambient temperature, a harder product may be manufactured, for example, by extraction of the lighter components with liquid propane or other solvents, or by oxidizing by air blowing at an elevated temperature, such as 275.degree. C. Both of these well established manufacturing processes can give asphalts that are too brittle at low temperature, and result in excessive cracking in cold weather.
Another means of stiffening soft asphalt to extend its useful temperature range is by blending with suitable polymers. Polymers can reduce the tendency of an asphalt pavement to creep and rut in warm weather by increasing its high temperature viscoelastic stiffness; and polymers can permit the use of softer grades of asphalt to minimize cracking in cold weather without risking excessive softening in hot weather. Unfortunately, many such asphalt-polymer blends are not storage stable.
When it is used as a pavement binder, asphalt is normally mixed with aggregate at a temperature where the asphalt is sufficiently fluid to coat the aggregate particles, typically 150.degree. C. or thereabouts. Asphalt is commonly stored, loaded, transported by truck or rail and pumped at a hot mix plant at similarly high temperatures so that it can be pumped and handled as a fluid. It is not always practical to keep an asphalt-polymer blend continuously homogenized before mixing the blend with aggregate; consequently, storage stability is desirable to keep the blend from separating under storage conditions.
However, not all asphalts and polymers form compatible mixtures. The storage stability of polymer modified asphalt is greatly affected by the compatibility of the asphalt with the particular polymer. At normal handling and storage temperatures, the mixture typically forms two liquid phases, with most of the polymer and the lighter, less polar asphalt components in one phase, and most of the heavier, more polar asphalt components in a second phase.
Various methods have been suggested for making polymer-modified asphaltic compositions that are sufficiently compatible to be acceptably storage stable and that also have the viscoelastic properties in the ranges required for a particular application. U.S. Pat. No. 5,348,994 (Gorbaty) teaches the preparation of storage stable hot mix pavement binder having improved viscoelastic properties inter alia by cosulfonation followed by neutralization of a blend of asphalt and polymer having some unsaturated bonds. Gorbaty introduces sulfonate groups to the polymer and asphalt, and adds neutralizing agent to the sulfonated polymer and sulfonated asphalt to form a storage stable blend. The neutralizing agent in Gorbaty could include amines, but in all instances, the neutralizing agent was a required component of the blend.